As a process for synthesizing an optically active AHPBN selectively, there is the process described in Tetrahedron Letters, Vol. 29, p. 3295, 1988.
This process, however, is disadvantageous in that the selective synthesis of the optically active AHPBN requires expensive reagents and a reaction condition of a very low temperature of -20.degree. C. or lower. Therefore, this process is not industrially suitable. On the other hand, a process comprising reacting an N-protected-L-phenylalaninal with sodium hydrogensulfite and potassium cyanide (EP-A-211580) can be practiced at ordinary temperature. This process, however, is poor in selectivity for an optically active reaction product, and no method for industrially easy separation of only the optically active reaction product has been known. Therefore, it can give the optically active reaction product only in low yield, and it is unavoidably expensive as a process for obtaining the optically active reaction product.